Single sideband radio transmitters are widely used in a range of radio communications applications. By way of example only, such transmitters are used by amateur radio operators, as well as by commercial radio communication systems and by the military. In all such applications of single sideband communication, it is desirable that the transmitter (including any linear amplifier used) be as free from distortion as possible. A system that introduces distortion may cause extraneous sidebands to be produced, creating interference with other services outside the normal radio channel, in addition to causing distortion of the transmitted signal.
Ideally, a single-frequency audio tone applied to the input of the transmitter should produce a single radio-frequency signal output from the transmitter. In a similar manner, two different audio tones, when applied simultaneously to the input of a transmitter, should produce only two radio-frequency signals out of the transmitter. However, in the presence of non-linearity in the transmitter, two discrete tones applied to the input may result in a variety of output radio frequencies, the amplitudes and frequencies of which are dependent upon the degree of non-linearity of the transmitter.
Heretofore, it has been proposed that two-tone generators may be employed for single sideband testing. Various devices for generating first and second audio tone signals having frequencies generally within the frequency range of human speech have been known and used heretofore. However, such testing as done prior to the present invention has been subject to limitations caused by difficulty in observing and interpreting an oscilloscope presentation of the transmitter output. For a variety of reasons recognizable to persons of appropriate skill in the applicable arts, such an oscilloscope display shows only relatively large amounts of non-linearity, and dependence upon the oscilloscope alone can result in the transmitting equipment being operated with significant non-linearity.